A careful study has been made of the effect of depletion of the fissile component of reactor fuel on the resonance component of the Doppler reactivity coefficient (DRC) for a lattice typical of a boiling water reactor (BWR). A parallel investigation has been carried out for both uranium- and thorium-based fuels. It is found that there are three principal effects, as follows, the first two of which tend to decrease the magnitude of the resonance component of the DRC and the third to increase it: direct competition of fission product absorption with that of the fertile isotopes overlapping of the fission product resonances with those of the fertile isotopes in uranium only, the formation of a large saturating resonance in 240Pu. As a result, in uranium-based fuels the resonance component of the DRC changes very little with depletion of the fissile isotope, while in thorium-based fuels there is a significant decrease in magnitude. Our results cannot be applied directly to a BWR since this would require consideration of the depletion history and void distribution over the entire core. The burnup selected for the uranium fuel was 35 000 MWd/ton, in line with current practice. In this material, effect 3 above is close to its maximum value while effects 1 and 2 increase with further burnup. Thus, it is also true that for extended burnup of uranium fuels, as are now being considered by the U.S. Department of Energy, the resonance component of the DRC is expected to decrease in magnitude.